1. Field of the Invention
The present invention relates to a micro machine or MEMS (Micro-Electro-Mechanical Systems) device using an actuator. For example, the present invention relates to a MEMS device using a piezoelectric type or electrostatic type actuator.
2. Description of the Related Art
The following bridge structure is preferably used in order to reduce a warp resulting from residual stress when switch and variable capacitor are formed using a MEMS device using a piezoelectric type actuator. According to the bridge structure, both ends of a beam (elastic member) where an electrode is arranged are supported by two fixed ends. In the variable capacitor having the bridge structure, if tensile residual stress exists in the beam, there is a problem that piezoelectric displacement decreases. In order to solve the foregoing problem, it is effective to employ a spring structure (flexure structure) as part of the beam.
However, the following problem arises as seen from the description of the following document 1 if the spring structure is symmetrically given on both sides of the electrode of the variable capacitor. Specifically, a restoring force of the electrode becomes weak from a state that electrodes contact each other. As a result, a failure resulting from stiction is easy to occur.
Document 1: K. F. Harsh et al., “The realization and design considerations of a flip-flop integrated MEMS tunable capacitor”, Sensors and Actuators 80 (2000) 108-118.
Even if the spring structure is employed, a fixed electrode may be further provided above the electrode to sufficiently secure the restoring force as seen from the description of the following document 2.
Document 2: D. Peroulis et al., “Electromechanical Considerations in Developing Low-Voltage RF MEMS Switches”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL 51, No. 1, January 2003.
However, according to the structure that is described in document 2, a cavity must be formed double, and in addition, the fixed electrode must be formed thick. For this reason, the following problem arises. Specifically, the process becomes complicated, and as a result, the cost increases.